Polyurethane-modified polyisocyanurate (PU-PIR) foam is a closed-cell rigid foam that has found great utility in the commercial roofing market. This material is used primarily as a component in roofing board due to its ability to provide superior insulation, mechanical strength, and capacity to satisfy certain building code regulations related to fire performance and structural integrity under intense heat.
The polymer comprising the foam structure contains a preponderance of polyisocyanurate chemical linkages formed from the self-reaction of polymeric polyisocyanate, yielding a 6-membered ring structure containing three isocyanates. In general, the polyisocyanurate group is known to impart relatively good flammability performance due to limited flame-spread propensity and low smoke generation inherent with the polymer. Specific industry tests used to assess flame-spread propensity and smoke generation include the FM E-84 tunnel and the UL-790 Spread-of-Flame tests. The polyisocyanurate group is also known to impart excellent thermal stability to the polymer since polyisocyanurate bonds generally remain intact and resist decomposition upon exposure to intense heat. Thermal stability is critical in order to meet industry standard regulations since structural integrity of the polymer and/or composite upon exposure to heat ultimately dictates the outcome of the test. Specific industry tests that assess structural stability under burning conditions include the Factory Mutual Research Center Construction Materials Calorimeter Standard #4470 (known as the FM calorimeter test) and the European Loss Prevention Council test.
Polyisocyanurate linkages are spaced by polyurethane linkages in PU-PIR foam. The polyurethane groups are formed simultaneously with the polyisocyanurate groups through reaction of polymeric polyisocyanate with polyol. The polyurethane linkages help reduce brittleness of the polymer, provide certain physical property enhancements, and contribute greatly to the overall processing ease of the reacting foam during the manufacturing process. However, polyurethane groups are generally known to detract from the flammability performance of the polymer due to their greater combustibility and increased smoke generation. Recently this issue has become increasingly important due to the more widespread use of flammable hydrocarbon blowing agents used to generate the foam's cellular structure. To a certain extent, overall flammability performance can be controlled through incorporation of certain classes of flame retardants which are generally known to be effective for these types of foams. Examples of such flame retardants include halogen-containing compounds, which are thought to interrupt flame propagation in the gas phase, and phosphorus-containing compounds, which are thought to help catalyze formation of a protective char layer upon exposure to a flame. Particularly useful and common flame retardants for this technology are compounds which combine the two elements, such as halogenated phosphate esters.
Incorporation of polyurethane bonds into a polyisocyanurate polymer matrix is also known to significantly reduce the thermal stability of the resulting polymer. The decomposition temperature of a typical polyurethane bond is approximately 100° C. lower than the typical polyisocyanurate linkage. Therefore, while incorporation of polyurethane into the polyisocyanurate structure provides several positive and essential benefits for this technology, performance in certain industry-standard tests requiring structural integrity upon exposure to heat may be compromised.